Crater lamp as a spotlight



Feb. 13, 1940. P. w. BLACKBURN I CRATER LAMP AS A SPOTLIGHT Filed Oct. 27, 193' r INVENTOR FMBZ/ZK m mull ATTORNEY L Patented Feb. 13,

TPATTENT OFFICE CRA'lElt LAMP AS A SPOTLIGHT Philip W. Blackburn, East Orange, N. 1., assignor to Westinghouse Electric & Manufacturing Company, East' Pittsburgh, Pa., a corporation Pennsylvania v Application October 27, 193'},- Serial No. 171,216

Claims.

The present invention relates to electrical 'discharge devices and has particular relation to a glow discharge lamp employing a point source of light from a luminous discharge which renders 6 the device readily adaptable to transferring electrical impulses to a visual record on film such as in television, picture transmission, and sound filin.

Devices of this type are known to the art but have heretofore been exceedingly inemcient in operation inasmuch as they have had numerous inherent disadvantages. For example, priorart lamps employed in television, picture transmis- 'sion, etc., either by wire or radio, have had a 5 life of but a few hours and have required a comparatively high voltage and current. Also there has been an entire lack of uniformity not only between different lampsbut between the spots produced by such lamps. Moreover, in

so these prior art lamps an increase in current does not give a proportional increase in intensity as is readily shown by plotting the modulation curve for such lamps.

It is accordingly an object of the present invention to provide a glow discharge device in which the discharge is confined to a restricted area of one of the electrodes so as to render the device readily adaptable to the transfer of electrical impulses to films such as employed in picture transmission, television and sound film.

Another object of the present invention is the provision of a glow, discharge device which is readily adaptable to the transfer of electrical impulses to films such as employed in television,

and sound film, wherein the device has a life of several hundred hours and is instantaneously responsive with fidelity to changes of signal current imposed between the electrodes at frequenciss within the range of the human ear.

Another object of the present invention is the provision of a glow discharge lamp which genera'tes a concentrated light spot of a wavelength lying partially within the invisible portions of the spectrum so as to readily sensitize photographic film and wherein a uniformity of modulation and other characteristics exists between the same and different lamps.

' A further object of the present invention is the provision of a glow discharge lamp readily adaptable to picture transmission which is stable in operation and operable at a low voltage and current.

Still further objects of my present invention will become obvious to those skilled in the art by reference to the A accompanying drawing where: V

Fig. 1 is an elevational view of a glow discharge lamp constructed in accordance with the present invention ,with parts thereof broken away 5 to better illustrate the device.

Fig. 2 is a cross-sectional view and on an enlarged scale of the device as shown in Fig. 1 prior to its being sealed to the enclosing envelope,

Fig. 3 is an elevational detailed viewof the 10 anode electrode support,

Fig. 4 is a top plan view of the device as shown in Fig. 2,

Fig. 5 is a schematic diagram showing a, circuit arrangement utilizing the glow discharge lamp of Fig. 1, and v Fig. 6 is a graphic illustration showing the relationship of light intensity to current.

Referring now more specifically to Fig. 1, the lamp as shown may comprise a vacuum tight envelope 5 which, after complete exhaustion is provided with a suitable gaseous filling, and having any conventional type base 6 provided with contacts so that electrical connection can be made from the electrodes to a suitable socket. 25 Extending inwardly of the envelope 5 is a reentrant press portion 8 of conventional design through which extends a pair of leading-in conductors 9 and ill connected, respectively, to the base 6 and contact tip l2. f 30 As shown more clearly in Fig. 2, the leadingin conductor ill, in cooperation with a rod l3 also sealed into the press portion 8 of'the envelope, supports an, anode electrode I4 which may take the form of an annular metallic thimble provided with a small aperture I5. This anode I may be of any suitable metal which can be readily cleaned and degassed, such for example as nickel, and may be secured to the leading-in conductor l0 and rod l3 by spot weld- 40 ing or by cutting the thimble and forming lugs It by winding or rolling the cut portion around the rod and leading-in conductor, as shown more clearly in Fig, 3.

A cathode ll, of a suitable metal having certain characteristics as hereinafter described, in the form of a rod is supported by the leadingin conductor 9. The upper end of the cathode i1 is spaced a definite distance of not greater 50 thanabout 2 mm. from the anode i4 and is provided with a recess in the form of a uniform crater I8 having a small diameter which, for example, may be of approximately .1 of an inch anda depth of approximately .15 inch, depend- M spot area.

Surrounding the cathode l| and resting on the reentrant press is a cylindrical shield 19 'of a comparatively good electrical insulating material which will preventelectrlcal leakage, such for example as porcelain or the like, but preferably lava, since the latter carrbe readily obtained in desired shapes and sizes. The upper open end of the shield l9, as sho wn in Fig. 2, may be provided with a small shoulder portion 20, so as to rest on.'the cathode and-insulate the same from the anode, the outer periphery of which may surround the shield. Y

In a lamp of this type several characteristics are essential, among them being a life of several hundred hours, uniformity of modulation, uniformity of light spot produced, uniformity between lamps, uniformity of breakdown voltage with low current flowand high energy output in that part of the spectrum lying between 3000 and-5000 Angstrom units in order to sensitize photographic films. To insure first a life of several hundred hours it has been found necessary to employ a stable gas ormixture of gases of suiiicient pressure so that clean-up of the gaseous filling will not occur. The gas most satv isfactorily meeting this condition was found to when filled with a mixture of 99.5% neon and be neon or preferably a mixture of gases containing a high percentage of neon at the minimum optimum pressure for cold cathode negative glow lamps of approximately 30 mm. which maintains minimum breakdown voltage.

It is known that a pure-inert gas, such as neon, has a definite breakdown voltage but in most cases this voltage is comparatively high. Also since it is essential in a lamp of this type that the breakdown voltage, 'as well as the voltage drop of the lamp, be kept low, the addition of from .2 to .5% argon to the neon lowers the breakdown and operating voltage by 30% or more than in the case of pure neon. For example,

alamp constructed in accordance with the present invention when filled with neon has a breakdown of approximately volts D. C. and a voltage drop of approximately 120 volts, but

.5% argon the breakdown voltage is lowered to approximately 100 volts D. C. and drop to approximately '75 volts.

Most photographic films; as before noted, are sensitive to radiations lying within the spectrum between 3000 and 5000 Angstromunits, and consequently the lamp must have an energy output which is comparatively high in this region. However, neon alone is very poor 'in this output and argon also is a low producer of these wavelengths. When admixed in the proportion of approximately 99.5% neon and .5% argon the spectrum changes so that it is richer in the yellow region but no better, if as good, in the blue and ultraviolet. It is preferable to therefore add, between .5% and 1% nitrogen to the above mixture while maintaining the minimum optimum pressure of approximately 30 mm. which changes the energy completely, and, as indicated by a spectrograph, the region between 3000 and 5000 Angstrom units which was very weak is now quite strong, inasmuch as the weak lines are much more pro nounced and many new lines are added. Upon exposing a photographic film for a very short period of time a complete sensitizing results.

Not only does the stability of the gaseous filling constitute a major factor in the life of the lamp, but in cooperation with the cathode it the voltage determines the uniformity between lamps while the cathode material and construction, as. well as its relative spacing with respect to the anode,

determines the uniformity of the light spot. In

order to obtain uniformity between the same and different lamps, there must be a definite breakdown voltage and voltage drop as well as a uniformity of spectral output, the latter of whlch is determined, as above noted, by the selection of the gas and its pressure. To obtain uniformity of breakdown and'voltage'drop' between the same and diflerent lamps it is essential that a cathode be employed having certain requisite characteristics as before mentioned.

For example, the cathode must have a high uniform cold emission and the material be a reservoir to, renew the emissive surface, as well as having a low work function, long life andlow sputter factor. Although clean nickel or other material coated with an emissive material such as barium azide will function, it has been found preferable to employ thorium, inasmuch as such a material has all the above noted characteristics which give uniformity of breakdown and voltage drop, and at thesame time gives a longer life since there is no coating which eventually sputters oil.

Uniformity of light spot is obtained-first by 0 providing the cathode with a uniform crater, the surface of which is smooth and clean. The craterv is cleaned and the high points removed by running the tube at high current and low gas pressure on exhaust which sputters off any contamination and high points that would otherwise cause non-uniformity during operation. Since the cathode is surrounded by the shield IS, the

procedure of cleaning the cathode naturally forms a deposit on the anode which. must also be cleaned. This is accomplished by operating the device at reversed polarity until the material is again sputtered off and removed from the envelope during exhaust.

Upon initial breakdown of the tube accompanied by minimum current flow the crater effect causes the formation of a spot on the surface of the crater at the lowermost portion thereof with the radiations being projected through the anode aperture. in the direction of the longitudinal axis of the lamp. The radiations thus form a projected spot of minimum intensity. When the current; is increased, as hereinafter described, the discharge'unifornily spreads over the smooth and clean cathode surface without .the formation of any dark areas and in proportion to the increase in current which causes a similar increase in the intensity of the projected spot.

Toprevent the formation'of a glow around the cathode except in the crater the cathode is carefully insulated, as before mentioned, by the insulating shield. I!) so that the crater is the nearest part electrically to the anode. In addition any so-called-fuzziness around the anode, due to a positive column discharge, is precluded by very carefully spacing the anode from the cathode at a distancele ss than 2 min.

By constructing a lamp in the manner described,-with.proper spacing of the electrodes,

proper gas pressure and mixture and proper cathode material, the lamp will have a uniformity in light spot and of breakdown and voltage volt D, C. source which is limited by a I resistance R to approximately 100 volts, breaks down since such voltage is no higher than that necessary to cause breakdown with attendant current flow of not more than 1.5 milliampei'es. This accordingly forms a light spot in the "crater" of the cathode with the radiation emanating through the aperture in the anode.

When a signal is received or the current increased by any means such as increased voltage or decreased resistance, it is amplified and, as noted in Fig. 5, the resulting increased voltage from the amplifier is impressed upon the input circuit for the lamp which then passes current in proportion to the increase in signal voltage. Since the voltage, prior to the receipt of any signal voltage, is just sufficient to cause breakdown of the tube and current flow, as above mentioned, is at a minimum, the intensity of the light spot produced is likewise at a minimum. However, as the signal voltage increases the current passed by the tube likewise increases. and since the useful light output is proportional to this increase in current, the light spot increases in intensity with fidelity.

This is shown by the curve of Fig. 6 wherein the abscissa represents current in milliamperes and the ordinate, output in intensity. It will thus be seen from this figure that not only does increase of light intensity follow with fidelity increases in current but the latter is a straight ascending line. Moreover, since the cathode crater is uniform, the spot is confined to a fixed point within the crater and its increase and decrease in intensity in accordance with modulation caused by the signal voltage completely sensitizes photographic film with a high degree of reproductive accuracy, since the light spot produces both visible and invisible radiations to which the film is sensitive.

It thus becomes obvious to those skilled in the art that by employing a cathode of a material having certain desired characteristics and spacing the same a definite predetermined distance from the anode and by proper selection of the gaseous filling and its pressure, a glow discharge lamp is produced which has a definite and comparatively low breakdown voltage and low current. Moreover, a light spot is produced which is confined to a fixed area of the cathode and the intensity of which modulates with fidelity in accordance with the signal voltage and with variations in current flow. Due to the comparatively lowvoltage and current together with the above noted requisites, the device has a relatively long useful life of several hundred hours, modulates uniformly, produces a uniform light spot giving a uniformity between the same and different lamps and the energy output is of a wave-length lying partly within the invisible portion of the spectrum which completely sensitizes photographic films Although I have shown and described one specific embodiment of my invention, I do not desire to be limited thereto, as various other modifications of the same may be made without departing from the spirit and scope of the appended claims.

I claim:

1. An electric discharge device comprising an envelope containing a gaseous mixture consisting of approximately 99% neon, and the remainder argon and nitrogen of at least 2% adapted upon excitation thereof to emit radia tions having its maximum intensity lying within the spectral range between 3000 and 5000 Angstrom units, an annular shaped metallic anode within said envelope provided with a reentrant portion forming a smooth surfaced aperture therein, and a cathode in axial alignment with said anode and spaced therefrom a distance no greater than 2 mm. to definitely limit the breakdown potential of said device to a preselected minimum, said cathode being of a metal having electron emissive properties commensurate with the barium group of elements and of a lower vapor pressure than such group of elements at the operating temperature of said device, antsaid cathode having a low work function and high cold electron emissive characteristics and provided with a crater therein having a clean, smooth surface and an arcuate end and a diameter at least no greater than the aperture in said anode.

2. An electric discharge device comprising an envelope having a gaseous atmosphere consisting of approximately 99% neon and the remainder argon and nitrogen, each of said gases argon and nitrogen being at least .2% of said gaseous atmosphere and cooperating electrodes within said envelope.

3. An electric discharge device comprising an envelope containing a gaseous mixture comprising at least 95% neon and the remainder argon and. nitrogen, said argon and nitrogen being present in a percentage greater than .2%, and cooperating electrodes within said envelope, and an insulator spacing said electrodes approximately 2 millimeters apart and an opening in one of, said electrodes.

4. An electric discharge device comprising an envelope having a gaseous atmosphere consisting of approximately 99% neon and the remainder argon and nitrogen, each of said gases argon and nitrogen being at least .2% of said gaseous atmosphere and cooperating electrodes within said 50 envelope, and an insulator spacing said electrodes approximately 2 millimeters apart and an opening in one of said electrodes.

5. An electric discharge device comprising an envelope having a gaseous atmosphere consisting of approximately 99% neon and the remainder argon and nitrogen, each of said gases argon and nitrogen being at least 2% of said gaseous atmosphere, and cooperating electrodes within said envelope, one of said electrodes having an opening therethrough and the other of said electrodes having a crater aligned with said opening and closely spaced thereto.

PHILIP W. BLACKBURN. 

